High voltage resistant members for outdoor service are known, which typically are made of fiberglass reinforced resin bonded rods surrounded by a suitable weather resistant material of a shape and construction which will shed rain. Metal fixtures are attached to the rods, for example in service as insulators, to allow connection to a conductor at one end and to a support structure at the other end.
A number of problems have been found in design, manufacture and application of high voltage resistant members. The problems in part are related to the nature of the weather resistant material surrounding the rod. Epoxy resins were among the earliest materials used as the weather resistant material and it was found that employment of hydrated alumina in large quantities improved their tracking and erosion resistance. This improvement, due to the hydrated alumina, also occurs with other polymers. The epoxy resin formulations which provide the best electrical performance, however, generally are rigid and subject to cracking at low temperatures, especially if the rod is loaded mechanically in tension or cantilever bending.
Various elastomers have been used to improve flexibility of the weather resistant material. These elastomers have included ethylene-propylene-diene monomer rubber (EPDM), ethylene-propylene monomer rubber (EPM), butyl rubber, silicone resins, fluorocarbon polymers and the like. EPM and EPDM are particularly attractive from a cost viewpoint. But the elastomers generally are formed by molding under pressure, though a few are castable. The castable elastomers usually suffer from inadequacies in this regard, such as poor tear strength, and usually are not able to incorporate sufficiently large quantities of hydrated alumina to give the needed performance in tracking and erosion resistance. The moldable elastomers require prohibitively large molds and presses if the high voltage members are to be molded in one piece.
A number of patents relating generally to the construction and manufacture of such high voltage resistant members are listed hereafter:
U.S. Pat. No. 1,991,700 Rost PA1 U.S. Pat. No. 2,683,185 Morrison PA1 U.S. Pat. No. 2,732,423 Morrison PA1 U.S. Pat. No. 2,945,912 Imhof PA1 U.S. Pat. No. 3,001,004 Black PA1 U.S. Pat. No. 3,001,005 Sonnenberg PA1 U.S. Pat. No. 3,118,968 Moussou PA1 U.S. Pat. No. 3,152,392 Coppack et al PA1 U.S. Pat. No. 3,328,515 Vose PA1 U.S. Pat. No. 3,291,899 Ward et al PA1 U.S. Pat. No. 3,356,791 McGowan PA1 U.S. Pat. No. 3,358,076 Rebosio PA1 U.S. Pat. No. 3,446,741 Hervig et al PA1 U.S. Pat. No. 3,531,580 Foster PA1 U.S. Pat. No. 3,544,707 Gamble PA1 U.S. Pat. No. 3,549,791 Yonkers PA1 U.S. Pat. No. 3,626,083 Minter et al PA1 U.S. Pat. No. 3,800,111 Holmstrom PA1 U.S. Pat. No. 3,898,372 Kalb PA1 British Pat. No. 816,926 Coppack PA1 British Pat. No. 902,197 Bannerman PA1 British Pat. No. 915,052 Sweetland PA1 British Pat. No. 1,066,209 Rebosio PA1 British Pat. No. 1,116,197 Rebosio PA1 British Pat. No. 1,182,045 Rebosio PA1 British Pat. No. 1,226,265 British Insulated
U.S. Pat. No. 3,898,372 discloses a method which circumvents the molding problem of EPM by molding each shed separately and then mounting the sheds over a fiberglass rod and filling the space between the sheds and rod with a silicone grease. This is a simple expedient allowing individually molded sheds to be produced at low cost. However, such a construction creates numerous potential access points for water invasion from the outside to the rod. When the rod becomes wet, it fails electrically. Such wetting could occur during high pressure water washing of the member, a practice used by many electric utilities to remove accumulated contamination.
British Pat. No. 1,182,045 seeks to eliminate the numerous joints by use of a preformed elastomeric sleeve. The internal surface of the sleeve is treated in order to render it bondable to the rod and its inside diameter must be sufficiently greater than the outside diameter of the rod to permit adhesive to be introduced in order to bond the rod and the sleeve together. Adhesive voids can result from such a cumbersome procedure.
U.S. Pat. No. 3,112,357 discloses a conductor with hardenable resinous compositions, but which are not particularly resistant to tracking and erosion when exposed to weather and contamination. Consequently a porcelain housing has been required for outdoor use. Such a porcelain housing is both heavy and fragile.